Problem with modifying table field with SQL

Question

In a particular table, there are some values that end with a null character (ascii(0)) and we don't want them null-terminated. I've been trying to use the following SQL statement to find those fields and then remove the null character from the end (each record has a 'field_id' field and the 'value' field which holds the actual value of that field_id):
update num_data
set value = substr(value,1,instr(value,ascii(0))-1)
where field_id = 'FIELD1'
select field_id
from num_data
where ascii(substr(value,length(value),1)) = 0
All it does is execute for several minutes before I just have to interrupt it, and no changes are made.

Answer 1

I see I messed up; the query should have been:
update num_data
set value = substr(value,1,instr(value,ascii(0))-1)
where field_id in
select field_id
from num_data
where ascii(substr(value,length(value),1)) = 0
But since I wrote this note, I ran that query again, and it worked as I wanted it to; maybe our DB was running slow at that time, though I waited a few minutes for results.
I appreciate you two taking the time to respond.